Up to now, an electric power steering device (EPS) that assists steering operation by a drive force of a motor is employed in a vehicle steering system. In the EPS, a brushless motor including multiple systems each having an inverter and a winding set whose energization is controlled by the inverter from the viewpoint of fail safe has been known (for example, PTL 1). In the motor of this type, when any system fails, the drive is continued by an electric power converter and the winding set in a normal system.
Incidentally, the motor of the EPS is mainly of a permanent magnet rotation type, and a powerful magnetic flux density needs to be obtained by a small permanent magnet configuring a rotor. For that reason, in the conventional art such as PTL 1, a neodymium magnet high in magnetic flux density is used as a permanent magnet.
A reduction in a coercive force of the neodymium magnet at high temperature has been known. The coercive force is a physical property of how a reverse magnetic field can be withstood without demagnetization when the reverse magnetic field is applied to the magnet. Hence, when a large current flows into the motor at high temperature, an excessive reverse magnetic field as compared with the coercive force is applied to the neodymium magnet, and the neodymium magnet is demagnetized (Irreversibly demagnetized). As a result, the characteristic of the motor changes with the result that a desired torque may not be obtained.
Hence, in the motor of the conventional art, a control needs to be performed to reduce an energizable maximum current at high temperature so that the neodymium magnet of the rotor is not demagnetized.
However, because the winding set of the motor is heated by energization, and the heat is radiated, the permanent magnet of the rotor is likely to become high temperature. In particular, the motor used for the EPS is likely to become high temperature by outputting an assist torque of continuous steering operation such as garage parking. Hence, in order to prevent the demagnetization of the neodymium magnet, a degree or a frequency of reducing a limit value of the energizable maximum current becomes high, resulting in a risk that a sufficient torque cannot be ensured. In particular, in the motor of the multiple systems as in PTL 1, it is a critical issue to ensure the torque of a normal system even at high temperature when a failure occurs in one system.
[PTL 1] JP-2011-78230 A (corresponding to US 2011/0074333)